1. Field of the Invention
This invention relates generally to a system for controlling a permanent magnet machine and, more particularly, to a system for providing real time current reference signals in the d- and q-axis for controlling the torque output of an interior permanent-magnet (IPM) synchronous machine that minimize the copper loss, where the system considers the inherent and non-linear magnetic saturation effects of the machine.
2. Discussion of the Related Art
IPM synchronous machines have become popular for certain automotive applications as a result of their high efficiency and extended speed range. These applications include use of the machines as an integrated starter/alternator in internal combustion engine vehicles and/or a traction motor in electric vehicles and hybrid vehicles. The IPM synchronous machine is a three-phase AC machine that includes three coils in the machine stator and a permanent magnet in the machine rotor. The stator coils receive three phase current command signals transformed from the d- and q-axis current command signals that are 90° out of phase with each other that control the torque and speed of the machine. For a particular torque command from the vehicle operator, the current command signals are selected to provide minimum copper loss for that torque. Further, the appropriate current command signals generate the maximum torque from the machine for certain operating conditions.
During vehicle operation, the current command signals applied to the IPM synchronous machine for a particular commanded torque or for a maximum torque have heretofore been provided from look-up tables. The look-up tables are generated from operating an equivalent machine on a dynamometer off-line under the various vehicle operating conditions, and storing the command signals that provide the best efficiency for a commanded torque. Generally, the current command signals are stored in the look-up table as d- and q-axis current reference values in the rotor reference frame, as is well understood in the art.
The look-up tables may provide the proper current command signals for laboratory conditions. However, the operating parameters of a vehicle change in response to temperature changes, vehicle aging, etc. Therefore, the look-up table values may not provide the optimum machine efficiency for a particular commanded torque during normal operation of the vehicle. It is therefore desirable to provide a technique for determining the current command signals applied to the IPM machine in real-time for the particular operating conditions of the vehicle.